In mining applications explosive compositions often need to be placed in boreholes/blastholes in defined depths and amounts. This helps ensure that the blast rock is broken down in the desired manner leaving rubble and muck pile or fragmentation which is as large as is manageable for more efficient and economical removal/further processing.
The results achieved by the blast are greatly affected by the size and shape of the actual blast rock. The practice of pre-splitting is commonly used to create relatively steep and smooth highwall faces for improved stability control over subsequent blasting. The pre-splitting process involves drilling a row of closely spaced boreholes along the line of the highwall which will only be filled with approximately one quarter to one third the explosive charge used in bench or production blastholes. The pre-splitting blasts should form cracks and fissures in the highwall without causing too much damage to the blast rock due to excess energy, also referred to as back break.
Since the boreholes drilled for pre-splitting are, in practice, the same size as those for production blasting the problem is created of how the correct, lesser, amount of explosives can be placed in the borehole at the correct depths for successful pre-splitting.
Currently, small columns of explosives known as decks are placed in the borehole and spaced from one another by the introduction of material such as gravel or drill cuttings or the use of introduced decking devices. In this manner the borehole is filled and portions of the explosives placed at the desired heights along its length. This is a relatively time consuming and labour intensive process involving the introduction of large quantities of filler material. Further, each deck of explosives generally has a primer or booster of some kind which must rest within or adjacent the explosive composition. Placement and maintenance of this device in each deck can be problematic since the ensuing filler can damage or re-locate the booster and effectively separate it from the charge.
There are further problems if the borehole is completely or partially filled with water, as can often happen. In this situation, after the explosive charge has been placed in the borehole, the booster is introduced and essentially dragged through the explosives in the hope that it will lodge sufficiently for successful later blasting. Even in stagnant water this process is less than ideal but in boreholes containing dynamic water it is extremely unreliable and the booster is frequently dislodged or the dynamic water may actually wash away the explosive charge before detonation can take place.
If the booster does not lodge successfully within the explosive charge or the charge is washed away then the result may be a misfire. This is a very costly process as the borehole will have to be cleared and possibly re-drilled. This is a serious drain on man hours and greatly increases the expense of mining an area.